Merge pull request #10319 from DougGregor/escalate-type-soundness-warning

Escalate the warning about the "== Self" type soundness hole to an error

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -1420,11 +1420,6 @@ ERROR(witness_self_same_type,none,
       " (in protocol %5) due to same-type requirement involving 'Self'",
       (DescriptiveDeclKind, DeclName, Type, DescriptiveDeclKind,
        DeclName, Type))
-WARNING(witness_self_same_type_warn,none,
-      "%0 %1 in non-final class %2 cannot be used to satisfy requirement %3 %4"
-      " (in protocol %5) due to same-type requirement involving 'Self'",
-      (DescriptiveDeclKind, DeclName, Type, DescriptiveDeclKind,
-       DeclName, Type))
 NOTE(witness_self_weaken_same_type,none,
       "consider weakening the same-type requirement %0 == %1 to a superclass "
       "requirement", (Type, Type))

lib/Sema/TypeCheckProtocol.cpp

@@ -3122,9 +3122,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
           auto proto = Conformance->getProtocol();
           auto &diags = proto->getASTContext().Diags;
           diags.diagnose(witness->getLoc(),
-                         proto->getASTContext().LangOpts.isSwiftVersion3()
-                           ? diag::witness_self_same_type_warn
-                           : diag::witness_self_same_type,
+                         diag::witness_self_same_type,
                          witness->getDescriptiveKind(),
                          witness->getFullName(),
                          Conformance->getType(),

test/Compatibility/self_same_type.swift

@@ -1,5 +1,10 @@
 // RUN: %target-typecheck-verify-swift -swift-version 3
 
+// Note: while Swift 3.2 originally intended to provide backward
+// compatibility here, the type-soundness issue was considered so severe
+// (due to it breaking the optimizer) that that we escalated it to an
+// error.
+
 protocol P {
   associatedtype T
 }
@@ -9,7 +14,7 @@ protocol Q {
 }
 
 class C1: Q {
-  func foo<T: P>(_: T, _: C1) where T.T == C1 {} // expected-warning{{instance method 'foo' in non-final class 'C1' cannot be used to satisfy requirement instance method 'foo' (in protocol 'Q') due to same-type requirement involving 'Self'}}}}
+  func foo<T: P>(_: T, _: C1) where T.T == C1 {} // expected-error{{instance method 'foo' in non-final class 'C1' cannot be used to satisfy requirement instance method 'foo' (in protocol 'Q') due to same-type requirement involving 'Self'}}}}
     // expected-note@-1{{consider weakening the same-type requirement 'T.T' == 'C1' to a superclass requirement}}{{41-43=:}}
 }